1. Field of the Invention
The invention relates to a process for regenerating a liquid from a spent liquid, in particular, black liquor from the kraft process for pulping wood, while simultaneously obtaining energy.
2. Description of the Related Art
In the generally known kraft process, fragmented wood is pulped with a liquid that consists essentially of soda lye, sodium carbonate, sodium sulfide, sodium sulfate and other salts in order to obtain cellulose. The liquid used is enriched with organic components, for example, carbon, oxygen and hydrogen. A liquid known as black liquor is created, the dry material of which consists of approximately 30% to 40% by weight of soda lye and salts and approximately 60% to 70% by weight of organic material.
According to the prior art, the black liquor is evaporated in a boiler apparatus until its dry material or matter is less than or equal to approximately 80% by weight and then burned in a substantially pressure-free environment, so that the organic components can be used for energy and the sulfur containing compounds removed in the most substantially reduced form possible. For this purpose, the lower part of the boiler apparatus, where the salt melt develops, is operated in substoichiometric fashion. The salt melt, which consists of Na.sub.2 CO.sub.3, Na.sub.2 S and Na.sub.2 SO.sub.4, is dissolved with weak wash, a liquid containing NaOH that is created during the kraft process, and is discharged as green liquor.
In the subsequent caustification process, approximately 80% to 85% of the sodium carbonate is converted into calcium hydrogen carbonate and soda lye. After separation of the calcium hydrogen carbonate, a liquid known as white liquor is obtained. This white liquor is essentially an aqueous solution of soda lye, sodium sulfide, sodium carbonate and sodium sulfate which may be reused for wood pulping. The necessary ingredients of the white liquor are soda lye and sodium sulfide.
The method according to the prior art is subject to several disadvantages. One disadvantage associated with such conventional kraft processes is that no reduction occurs during the combustion of black liquor in the upper part of the boiler, which is operated in superstoichiometric fashion. As a result, the flue gas contains SO.sub.2 and Na.sub.2 SO.sub.4 pollutants which must be removed by expensive flue-gas purification measures, for example, flue-gas desulfurization and electric filters, before being emitted into the atmosphere. Furthermore, the salt melt also contains sodium sulfate and sodium thiosulfate, which are not removed in the subsequent processing steps and--since they are not needed during wood pulping--circulate as ballast or material in the kraft process.
Furthermore, during the combustion of black liquor at the usual temperatures of approximately 900.degree. C. to 1000.degree. C., only a negligible share of soda lye is created in the salt melt. However, soda lye is vital for the process of wood pulping. A majority of the required soda lye is obtained during the caustification process that follows combustion, and the rest must be added in the form of additional NaOH.
Another disadvantage of the kraft process in accordance with the prior art technique is that the chemically-bound heat of the black liquor is converted into steam at low boiler efficiency thereby making it extremely difficult, if not impossible, to produce a combustible gas.
World Intellectual Property Organization patent publication 93/02249 describes a process for the pyrolysis and partial combustion of liquids containing sodium compounds. In this process, pyrolysis and partial combustion occur in an uncooled, lined reaction chamber. This process is disadvantageous in that the lining material is damaged by the liquid melt created during the process as well as by the sulfurous crude gas, thereby reducing the serviceable life of the reactor lining. In addition, the subsequent removal of dry slag or cinder entails greater apparatus-related expense for separating the produced gas and salt particles and also raises the risk of caking and incrustation in the gas channel and salt remover.
European patent publication 0 459 962 A1 discloses a process that uses a quenching system for cooling the crude gas, so that the salts can be removed from the reactor as a dissolved solute. This proposed process has the disadvantage that, because of the longer contact time of the salts with the H.sub.2 O, CO.sub.2 and H.sub.2 S in the crude gas prior to dissolution in water, chemical reactions result in undesirable carbonate formation. Furthermore, it is possible to obtain soda lye by gasification only to a limited extent, if at all.
Thus, all known processes for the thermal recovery of green liquor from black liquor have the disadvantage that either not enough Na.sub.2 CO.sub.3 is decomposed and not enough NaOH is formed during the thermal treatment, and/or that the formed soda lye reacts during the direct gas cooling with CO.sub.2 and carbonate is again formed. As a result, the required NaOH must either be obtained by caustification of Na.sub.2 CO.sub.3 or added to the kraft process as additional NaOH.